Growing marine fish has been generally limited to costal regions or seawater tanks. However, many freshwater aquifers exist, for example, in the Midwest as potential environments for the raising of marine fish. Until now, attempts to grow marine fish in freshwater have been unsuccessful.
Growing marine fish in freshwater would provide an opportunity for non-costal areas to raise marine fish. The ability to grow marine fish in freshwater can provide fresh fish and economic growth to these areas.
Hence, a need exists to determine whether it is possible to adapt a marine fish to freshwater, and if so, understand the biological mechanisms that allow a marine fish to do so. In particular, a need exists to grow marine fish in freshwater.
The present invention relates to methods of growing marine fish in freshwater by increasing or maintaining expression of a receptor, referred to as the Polyvalent Cation Sensing Receptor (PVCR). The expression and/or sensitivity of the PVCR is increased or maintained by subjecting the marine fish to at least one modulator of the PVCR. The marine fish are subjected to the modulator when it is added to the freshwater environment, and optionally, to the feed. The invention encompasses adding at least one PVCR modulator to the freshwater, and adding feed for fish consumption to the freshwater. The feed contains sodium chloride (NaCl) and, optionally, at least one PVCR modulator in amounts to contribute to a significant increased level of the PVCR modulator in the serum of the marine fish.
In one embodiment of the invention, marine fish (e.g., Cod, Haddock, Hake, Halibut, Mackerel, Pollock, Sea Bass, Swordfish, Tuna, Winter Flounder and Summer Flounder) are transferred to freshwater to which at least one PVCR modulator (e.g., a PVCR agonist), such as calcium and magnesium has been added. A specially developed or modified feed is added to the freshwater for fish consumption. The feed has between about 1% and about 10% NaCl by weight (e.g. between about 10,000 mg/kg and 100,000 mg/kg). The amount of calcium added to the freshwater is an amount sufficient to bring the concentration up to between about 2.0 mM and about 10.0 mM, and the amount of magnesium added is an amount sufficient to bring the concentration up to between about 0.5 mM and about 10.0 mM. The feed can optionally include a PVCR modulator, such as an amino acid. A particular amino acid that can be added is tryptophan in an amount between about 1 gm/kg and about 10 gm/kg. The present invention also includes, optionally, exposing the marine fish to a photoperiod. Preferably, the photoperiod is continuous (e.g., for a continuous period of between about 12 hours and about 24 hours in a 24 hour period).
In another embodiment, the invention relates to methods of growing marine fish in freshwater by assessing the freshwater to determine the level of at least one PVCR modulator. Based on the level determined, the method includes adding enough PVCR modulator, if any, to the freshwater in an amount sufficient to increase or maintain expression and/or sensitivity of at least one PVCR. The methods include transferring the marine fish to the freshwater, and adding feed for fish consumption to the modified freshwater, as described herein.
The present invention also relates to an aquatic mixture for providing an environment for transfer of marine fish to freshwater, comprising at least one PVCR modulator. Similarly, the present invention includes kits for growing marine fish in freshwater. The kits include the aquatic mixture described herein, and an aquatic food composition containing a concentration of NaCl between about 10,000 mg/kg and about 100,000 mg/kg.
Surprisingly, it has been discovered that increased or maintained expression and/or altering the sensitivity of the PVCR allows these marine fish to live and thrive in freshwater. Until the discovery of the present invention, the aquaculture industry was unable to transfer the marine fish to freshwater without subjecting the fish to stress, death and/or disease. Unlike this practice, carrying out the steps of the invention increases or maintains the expression and/or alters the sensitivity of the PVCR and allows for transfer of the marine fish to freshwater with minimal or no stress, death and/or disease, and unexpectedly, the fish grow. In fact, marine fish that grow in freshwater have a higher fat content, and a milder, less xe2x80x9cfishyxe2x80x9d taste.